Enzymic depolymerization of macromolecular heparin as a factor in control of lipoprotein lipase activity

Rat heparins. A study of the relative sizes and antithrombin-binding characteristics of heparin proteoglycans, chains and depolymerization products from rat adipose tissue, heart, lungs, peritoneal cavity and skin

35S-labelled heparins were recovered from adipose tissue, hearts, lungs, peritoneal cavities and skins of rats given H2(35)SO4. Their purification involved incubation with Pronase, precipitation with cetylpyridinium chloride in 1.0 M-NaCl, gradient elution from DEAE-Sephacel and incubation with chondroitinase ABC. Each product was divided into proteoglycan and "depolymerization products' fractions by gel filtration on Bio-Gel A-15m. Heparin chains were released from a portion of each proteoglycan fraction by beta-elimination with NaOH. Proteoglycans, chains and depolymerization products were separated by gradient elution from a column of antithrombin-agarose into fractions with no affinity, low affinity and high affinity for antithrombin. The relative sizes of the products were determined by gel filtration on columns of Bio-Gel A-50m, A-15m, A-1.5m and A-0.5m. Skin was the major source of heparin and contained the largest proteoglycans and the lowest proportion of depolymerization products. Lungs contained the smallest proteoglycans, the smallest depolymerization products and the highest proportion of depolymerization products. The highest proportions of proteoglycans, chains and depolymerization products with high affinity for antithrombin were found in adipose tissue. The lowest proportions of each of these fractions were found in the peritoneal cavity. The data suggest that there was relatively little biosynthesis of sites with high affinity for antithrombin in peritoneal-cavity mast cells and that heparin catabolism was most active in lungs. Each source of heparin was unique with respect to both biosynthesis and subsequent breakdown of its proteoglycans.

Circulating heparan sulfate proteoglycan anticoagulant from a patient with a plasma cell disorder

A woman, aged 68, with multiple myeloma (immunoglobulin[Ig]A kappa type) developed an anticoagulant with properties suggestive of heparin. The anticoagulant prolonged the thrombin time but not the reptilase time and was resistant to boiling, proteolytic enzyme digestion, and trichloracetic acid precipitation. The thrombin time was corrected by the addition (in vitro) of protamine sulfate or the addition of purified platelet Factor 4 (PF4) to the plasma. The anticoagulant was isolated by PF4-Sepharose affinity chromatography and analyzed in terms of its molecular weight, uronic acid, and amino acid composition. The proteoglycan isolated had a mol wt of 116,000 and appears to consist of two 38,000 dalton polysaccharide units interconnected by peptide material totaling 39,000 daltons. Electrophoretic analysis of the pronase digested peptidoglycan using the lithium acetate-agarose technique suggested the material was of the heparan sulfate type. The peptidoglycan had about one-tenth the specific activity of commercially available heparin on a weight basis. The isolated proteoglycan was indistinguishable from commercial heparin when analyzed in terms of its ability to act as a cofactor in the antithrombin III inhibition of thrombin.

The assay and partial characterization of macromolecular heparin depolymerase activity in rat small intestine

Homogenates of rat small intestine can depolymerize macromolecular rat skin heparin (RS heparin) to products similar in size to commercial heparin [Horner (1972) Proc. Natl. Acad. Sci. U.S.A. 69, 3469--3473]. This activity is attributed to an enzyme provisionally named 'macromolecular heparin depolymerase'. An assay for macromolecular heparin depolymerase activity in rat small intestine has been developed, based on the action of the enzyme on 35S-labelled macromolecular RS heparin. The depolymerized products are separated into two peaks by gel chromatography through columns of Bio-Gel A-15m. The amount of label in the second peak, expressed as a percentage of the total radioactivity, is the index of enzyme activity. The pH optimum was found to be 6.0 and the temperature optimum 45 degrees C. The enzyme was shown to be most stable in 50mM-Tris/maleate buffer containing 1 mM-EDTA. Macromolecular heparin depolymerase activity measured as a function of time and substrate concentration produced curves typical of an enzymic reaction. Evidence was obtained demonstrating that the activity did not originate from bacteria in the intestine. Macromolecular heparin depolymerase activity was increased by dilution and storage at 7 degrees C for 24 h. This suggests that homogenates of rat small intestine contain an unstable inhibitor of the enzyme.

The molecular-weight-dependence of the anti-coagulant activity of heparin

It is proposed that the anti-coagulant activity of heparin is related to the probability of finding, in a random distribution of different disaccharides, a dodecasaccharide with the sequence required for binding to antithrombin. It is shown that this probability is a function of the degree of polymerization of heparin. The hypothesis has been been tested with a series of narrow-molecular-weight-range fractions ranging from 5,600 to 36,000. The fractions having mol.wts. below 18,000 (comprising 85% of the original preparation) followed the predicted probability relationship as expressed by the proportion of molecules capable of binding to antithrombin. The probability that any randomly chosen dodecasaccharide sequence in heparin should bind to antithrombin was calculated to 0.022. The fraction with mol.wt. 36,000 contained proteoglycan link-region fragments, which may explain the deviation of the high-molecular-weight fractions from the hypothetical relationship. The relationship between anti-coagulant activity and molecular weight cannot be explained solely on the basis of availability of binding sites for antithrombin. The activity of high-affinity heparin (i.e. molecules containing high-affinity binding sites for antithrombin), determined either by a whole-blood clotting procedure or by thrombin inactivation in the presence of antithrombin, thus remained dependent on molecular weight. Possible explanations of this finding are discussed. One explanation could be a requirement for binding of thrombin to the heparin chain adjacent to antithrombin.

Metabolism of macromolecular heparin in mouse neoplastic mast cells

1. Polysaccharide in a heparin-producing mouse mastocytoma was pulse-labelled in vivo with [35S] sulphate, and after various periods of time was isolated from subcellular fractions. Such fractions were recovered from tissue homogenates by consecutive centrifugations at 1000g for 10min, 20000g for 20min and 100000g for 1h. Initially the 35S-labelled polysaccharide formed occurred principally in the second centrifugal fraction (20000g precipitate), with small amounts in the first (granular) and third (microsomal) fractions. Analysis for glycosyltransferase activity confirmed that glycosaminoglycans were formed chiefly in particles sedimenting at 20000g. Molecules of this newly synthesized polysaccharide were considerably larger than those of commercially available heparin, as judged from gel chromatography. 2. Within the first hour after injection of [35S]sulphate, most of the labelled polysaccharide was redistributed from the second to the first centrifugal fraction. During, and possibly also after, this shift, the macromolecular polysaccharide was degraded, ultimately to the size of commercial heparin. The degradation process appeared complete 6h after injection of [35S]sulphate. 3. Particulate subcellular fractions were incubated with macromolecular [35S]heparin and the products were analysed by gel chromatography. Significant degradation of the substrate occurred only with the second centrifugal fraction. Further characterization of this fraction, by density-gradient centrifugation in iso-osmotic colloidal silica, revealed a single visible band of particles, at approximately the same density at lysosomes. This band contained all the beta-glucuronidase, 35S-labelled endogenous polysacchride and heparin-degrading enzyme present in the second fraction.

Macromolecular properties and end-group analysis of heparin isolated from bovine liver capsule

Glycosaminoglycans were extracted from bovine liver capsule with 4 M-guanidinium chloride, resulting in solubilization of approx. 90% of the total uronic acid-containing polysaccharide of the tissue. The extracted polysaccharide was purified and fractionated by anion-exchange chromatography on DEAE-cellulose, density-gradient ultracentrifugation in CsCl and finally gel chromatography on Sepharose 4B. By using these procedures, the two major polysaccharide components, dermatan sulphate and heparin, which constituted 55 and 30% respectively of the total glycosaminoglycan content of the tissue, were separated from each other. Analysis of the macromolecular properties of the two polysaccharides showed that heparin existed exclusively as single polysaccharide chains, whereas dermatan sulphate occurred largely as a proteoglycan (protein content, 74% dry wt.). The purified heparin preparation was subjected to sedimentation-equilibrium ultracentrifugation, indicating a molecular weight of 8800. Analysis for neutral sugars (by g.l.c.) showed 0.1 residue of xylose and 0.2 residue of galactose/polysaccharide chain; serine amounted to 0.3 residue/polysaccharide chain. Reduction of the heparin with NaB3H4 resulted in incorporation of 3H, approximately corresponding to one reducible group/polysaccharide chain. The 3H-labelled sugar residue was liberated by a combination of acid hydrolysis and deaminative cleavage of the polysaccharide with HNO2; it was subsequently identified as an aldonic acid by paper electrophoresis. Most of the heparin chains thus contained a uronic acid residue in reducing position. It is suggested that heparin isolated from bovine liver capsule is a degradation product released from larger molecules by an endo-glycuronidase.

Crystallization of macromolecular heparin

X-ray fibre-diffraction photographs were obtained from oriented films of the sodium salt of macromolecular heparin (molecular weight approx. 10(6)) prepared from rat skin. Two distinct molecular chain conformations corresponding to two different crystal lattices were observed as a function of relative humidity. The first conformation, obtained at 78% relative humidity, has a layer-line spacing of 1.73nm, which can be interpreted as an approximate twofold helix. On increasing the relative humidity to 84% a second phase with a layer-line repeat of 1.65nm is obtained with the reflexions indexing on a triclinic unit cell similar to that obtained previously (Nieduszynski & Atkins, 1973) for pig mucosal heparin.